The present invention relates to a multi-beam scanning device that is to be utilized in a laser beam printer, a laser photo-plotter, or the like. In the multi-beam scanning device, a plurality of laser beams are deflected by a rotating polygonal mirror and passed through an fθ lens system. Each laser beam passed through the fθ lens converges in a vicinity of an object surface, such as a surface of a photo-sensitive drum or a photo-surface of a printed circuit board, to form a spot that scans the object surface in a main scanning direction at a constant scanning rate. The object surface is moved at a constant speed in a direction perpendicular to the main scanning direction, or an auxiliary scanning direction, so that a plurality of scanning lines are formed on the object surface. The laser beams are scanned across the object surface while being modulated on/off in accordance with an image information to form a two dimensional image on the object surface.
Typically, in order to correct the so-called tilting error (facet error) of the polygon mirror, the fθ lens has a power in the auxiliary scanning direction that makes the reflecting surface of the polygon mirror to be optically conjugate to the object surface. Further, a cylindrical lens is provided between the laser source and the polygon mirror so that each laser beam is converged in the auxiliary scanning direction near the polygon mirror.
Some multi-beam scanning devices are provided with an additional deflector for moving in parallel the laser beams traveling toward the polygon mirror. The additional deflector controls the positions of the laser beams in auxiliary direction so that the scanning lines are formed on the object surface at a constant interval even if the moving speed of the object surface in the auxiliary scanning direction varies.
It should be noted that if the fθ lens system is designed to correct the tilting error of the polygon mirror, the focal length of the fθ lens in the auxiliary scanning direction is relatively short, resulting in a large field curvature, or Petzval curvature, in the auxiliary scanning direction. Thus, if the additional deflector shifts the laser beam for a large distance in the auxiliary direction, the beam waist will be largely displaced from the object surface and the spot formed on the object surface becomes to have a size exceeding an acceptable size range. Therefore, the additional deflector is typically designed to move the laser beams only in a vicinity of the optical axis of the fθ lens.
If one of the lens in the optical system of the multi-beam scanning device is decentered due to a manufacturing error or inclined due to an assembling error, the image plane of the optical system, which has large field curvature in the auxiliary direction, inclines against the object surface. The inclination of the image plane may occur such that the image plane on one side, or first side, of the optical axis in the auxiliary scanning direction displaces more than on the other side, or second side.
The inclination of the image plane described above increases, on the first side of the optical axis, the displacements of the beam waist positions caused by the additional deflector shifting the laser beams in a direction from the second side of the optical axis to the first side. The increase of the displacement becomes larger with the distance of the laser beams from the optical axis. Thus, the displacement of the beam waist position of the laser beam most apart from the optical axis becomes extremely large when the additional deflector fully shifts the laser beams. As a result, the size of the spot formed on the object surface by the laser beam most apart from the optical axis becomes much larger than that formed by the laser beam on the second side or near the optical axis. This causes an unacceptable size difference among the spots formed on the object surface, which deteriorates the imaging quality of the multi-beam scanning device.
The inclination of the image plane, and hence the size of the spot formed on the object surface, can be corrected by reshaping the decentered lens or reassembling the optical system. Such methods, however, require much labor hour, which decreases the production efficiency of the multi-beam scanning device.
Therefore, there is a need for a multi-beam scanning device that is capable of preventing an unacceptable size difference among the spots formed on the object surface caused by the manufacturing and/or assembling error of the lens in the optical system without requiring reshaping the lens or reassembling the optical system.
Generally, the multi-beam scanning device requires adjustment of the optical system thereof so that the laser beams are converged in a vicinity of the object surface in both the main scanning direction and the auxiliary scanning direction. Such adjustment is carried out by moving two lenses in the optical axis direction. The first one is a rotationally symmetrical collimator lens provided for converting a divergent light beam emitted from a laser source into a parallel light beam. The second one is the cylindrical lens provided for correcting the tilting error of the reflecting surface of the polygon mirror. The collimator lens is moved to adjust the position at which the laser beam is converged in the main scanning direction, while the cylindrical lens is moved to adjust the position at which the laser beam is converged in the auxiliary scanning direction.
However, if the collimator lens is moved, the light beam entering the additional deflector, which is placed between the collimator lens and the cylindrical lens, becomes a divergent or converging light. If the additional deflector is operated in such a condition, the image plane position displaces in the optical axis direction and the sizes of the spots formed on the object surface change. The displacement of the image plane, and hence the change in spot size, increases as the additional deflector increases the laser beam shifting distance. Since the additional deflector changes the laser beam shifting distance in accordance with the variation of the moving rate of the object surface, the size of the spots formed on the object surface also changes in accordance with the moving rate of the object surface, which causes deterioration of the imaging quality of the multi-beam scanning device.
Therefore, there is a need for a multi-beam scanning device that is capable of adjusting the converged positions of the laser beams in each of the main scanning direction and the auxiliary scanning direction while avoiding the laser beam entering the additional deflector becoming a diverging or converging light.